In underwater laying and retrieving of cable or pipelines in addition to instrument, oceanographic sensor, or other arrays useful for seismic survey, oceanographic data gathering, or other purposes, it is sometimes necessary to know the exact location of the cable, pipeline or array (which may be called an object) to avoid collisions, obstacles, achieve precise location or intersections, or for any number of other reasons.
A transponder is any active electronic device that receives a signal at one frequency and in response thereto transmits a reply signal. Radar transponders are used in aircraft for identification in congested airspace. Acoustic transponders (typically operating in the range from 7 kHz to 16 kHz) are used for underwater ranging and locating.
Acoustic transponders can be attached to the object for locating using a scheme similar to that disclosed in U.S. Pat. No. 3,860,900 granted to Ronald J. Scudder on Jan. 14, 1975. In this way, a given section of cable (at a given transponder) can be accurately positioned or located with respect to the cable-laying/retrieving vessel and with respect to the fixed transponders of the Scudder patent that are accurately located on the ocean bottom.
However, with the advent of thinner, lighter-weight, more-critically-designed objects, such as light-guide communication cable or other light-weight objects, more information is needed than just the location of a section of an object. For example, newer underwater cables are not quite so robust as older styles of cable; and this delicacy of structure places more critical requirements on the conduct of cable handling. Consequently, as a cable is positioned or retrieved, feedback is important to avoid placing undue tension on the cable and thus damaging it.
It would be desirable to attach strain gauges to the cable and transmit the output from each strain gauge to the cable ship, using acoustic telemetry. However, with all of the acoustic signals involved in using the locating system of the Scudder patent, especially at a substantial depth, the use of still more acoustic signals for strain gauge telemetry might risk confusion from acoustic clutter or might slow the gathering of real-time data, to an unacceptable degree.
The signals sent to the transponder and transmitted by the transponder are short bursts or pulses of tone energy at the frequency selected for the signal. Because of the slow speed of sound, even in water, the transit times of each signal can be many seconds. Multipath signal propagation is the rule rather than the exception in the undersea environment. Therefore, a signal can appear much longer at the receiver than when transmitted. Consequently, signal pulse duration cannot be used as a realistically accurate medium for telemetry, but has been found useful in a gross sense to indicate the existence of binary conditions such as the successful release of a transponder from its anchor to a buoyed tether.